Disclosed herein is a variant version of the antigenic substance, Streptotysin O. Streptolysin O is associated in humans with, for example, rheumatic fever, such that immunodiagnostic assays for evidence of immunological response against Streptolysin O are routinely utilized. The disclosed variant version of Streptolysin O is produced by recombinant DNA techniques, is soluble upon expression, and has substantially no hemolytic activity. Prior to this invention, Streptolysin O could be obtained via the bacteria Streptococcus pyogenes. However, Streptolysin O, in its wild-type form, is a dangerous substance in that it will lyse cells, lyse neutrophils, inhibit lymphocyte transformation, and release histimine from platelets. The toxic and pathogenic properties of Streptolysin O are typically monitored by the lysis of red blood cells.